This invention generally relates to the imaging of moving ultrasound scatterers. In particular, the invention relates to methods for tracking the position and/or orientation of a blood vessel in medical diagnostic ultrasound imaging.
Premium medical diagnostic ultrasound imaging systems require a comprehensive set of imaging modes. These are the major imaging modes used in clinical diagnosis and include timeline (spectral) Doppler, color flow Doppler, B mode and M mode. In the B mode, such ultrasound imaging systems create two-dimensional images of tissue in which the brightness of a pixel is based on the intensity of the returned echo. Alternatively, in a color flow imaging mode, the movement of fluid (e.g., blood) or tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The phase shift of backscattered ultrasound waves may be used to measure the velocity of the backscatterers from tissue or blood. The Doppler shift may be displayed using different colors to represent speed and direction of flow. In the spectral Doppler imaging mode, the power spectrum of these Doppler frequency shifts are computed for visual display as velocity-time waveforms.
One of the primary advantages of Doppler ultrasound is that it can provide noninvasive and quantitative measurements of blood flow in vessels. Given the angle xcex8 between the insonifying beam and the flow axis (hereinafter referred to as the xe2x80x9cDoppler anglexe2x80x9d), the magnitude of the velocity vector can be determined by the standard Doppler equation:
v=cfd/(2f0 cosxcex8)
where c is the speed of sound in blood, f0 is the transmit frequency and fd is the motion-induced Doppler frequency shift in the backscattered ultrasound signal.
In conventional ultrasound scanners that perform B-mode and spectral Doppler imaging either simultaneously or in a segmented fashion, the angle between the Doppler beam cursor (beam centerline) and a vessel slope cursor in the B-mode image is used to convert Doppler frequency shifts into velocity units according to the Doppler equation. The operator is required to manually adjust (e.g., via a toggle switch) the vessel slope cursor based on the orientation of the vessel wall(s) in the B-mode image. The Doppler angle value is usually displayed along with the graphic. Since the Doppler angle adjustments are based on visual judgment, they are susceptible to error, especially if the angle step size is coarse. If fine angle adjustments are possible, the process can become time consuming.
An automatic method of adjusting the vessel slope cursor is taught in U.S. patent application Ser. No. 09/201,982, entitled xe2x80x9cMethod and Apparatus for Automatic Doppler Angle Estimation in Ultrasound Imagingxe2x80x9d. The Doppler angle is estimated automatically based on the B-mode and color flow (if available) image. The method uses an algorithm for automatic vessel slope measurement which first finds an optimal initial point within the sample volume or range gate, and then searches for the most reliable pixel points (near or far wall) based on a combination of intensity-only and intensity-difference thresholds, before performing a slope estimation. B-mode intensity data and, optionally, color flow velocity or power data (before gray/color mapping) are used. The algorithm may also be applied to methods for automatic tracking of vessel diameter and flow rate calculations.
In clinical ultrasound imaging studies, sometimes it is necessary to examine the blood flow at multiple sites along a segment of a blood vessel. With conventional scanners, if the vessel depth changes as the probe is moved along the vessel, the range gate needs to be maintained inside the vessel by manual control (e.g., using a trackball). When examining a flow at a pre-selected vessel position, the range gate can slide off of the vessel and may sometimes even shift outside the vessel due to tissue and/or probe motion.
Thus there is a need for a method by which the range gate can be automatically maintained inside a moving blood vessel being examined.
The present invention is directed to a method and an apparatus for automatically maintaining the range gate inside a moving blood vessel during ultrasound imaging. The range gate can be maintained at the vessel center, or at a certain distance from one of the vessel boundaries, or at a certain ratio of the respective distances from the two boundaries.
In accordance with the method of the preferred embodiment, the user first places the range gate on the blood flow being examined. Instead of pushing a control key or button to activate the automatic Doppler angle estimation based on a particular B-mode image frame (and color flow data if available), an algorithm processes each successive image frame and automatically updates the information about vessel boundary positions and the vessel orientation angle in the vicinity of the range gate. If the range gate position relative to the vessel boundaries is different than that in the previous frame by more than a predetermined amount, the position of the range gate graphic and the orientation of the vessel slope cursor (and the Doppler angle) are automatically adjusted to the new values. Optionally, the steering angle of the transmitted beam can also be automatically updated.